The problem of migrating particles of highly absorbent material has been considered in several publications but to the knowledge of the inventors it has not yet been satisfactorily resolved. For example U.S. Pat. No. 4,321,924 discloses a sanitary napkin which comprises a fibrous layer as part of the topsheet to improve strike through characteristics of the absorbent article. However no disclosure of the appropriate size of pores in the topsheet is given and the problem of particle migration in absorbent structures is not touched upon.
U.S. Pat. No. 4,752,349 discloses a sanitary napkin wherein a wicking layer adjacent to the topsheet extends beyond the periphery of the absorbent core. It therefore would serve as a barrier along a straight line between particles in said absorbent core and the liquid passage ways in the topsheet of the sanitary napkin. However no barrier against migration around the edge of the tissue is foreseen or considered since this disclosure does not relate to the same problem underlying the present invention.
U.S. Pat. No. 5,009,653 shows absorbent gel material containment structures interposed between topsheet and backsheet of absorbent articles. This patent addresses the problem of particle migration from the absorbent structure through the liquid passage ways in the topsheet and implies that a wetlaid tissue between the topsheet and the absorbent structure would reduce absorbent gel material particles migration. However as in the above cited prior art no shielding at the edge of the wetlaid tissue is provided for.
EP-A-248 584 also discloses sanitary napkins which have fully enclosed absorbent structures by providing several layers of tissue around the particle containing absorbent structure. However these tissues are not joined to each other along an endless peripheral line such that particle migration paths remain open for particles to ultimately migrate through the liquid passage ways in the topsheets employed in these articles.
It is therefore an objective of the present invention to fully shield absorbent structures comprising absorbent particles from the liquid passage ways in the topsheet of absorbent articles. In particular tissues having pore sizes smaller than the smallest particle are desirable since they simultaneously perform as wicking and distribution layers while serving as particle migration shields.